A battery is a device that converts chemical energy to electrical energy. The battery is a combination of one or more electrochemical cells, each cell consists of two half-cells connected in series by a conductive electrolyte. One half-cell includes electrolyte and an electrode to which negatively-charged ions migrate, for example the anode or negative electrode. The other half-cell includes electrolyte and an electrode to which positively-charged ions migrate, for example the cathode or positive electrode. The electrodes do not touch each other but are electrically connected by the electrolyte. Many cells use two half-cells with different electrolytes. In this configuration, each half-cell is separated by a separator. The separator is porous to ions, but not the electrolytes, thereby enabling ions to pass but preventing mixing of the electrolytes between the two half-cells.
A battery explosion may occur through misuse or malfunction of the battery, such as attempting to recharge a non-rechargeable battery or short circuiting a battery. When a battery is recharged at an excessive rate, an explosive gas mixture may be produced leading to pressure build-up and the possibility of the battery case bursting. Overcharging, which occurs when attempting to charge a battery beyond its electrical capacity, can also lead to a battery explosion, leakage, or irreversible damage to the battery.
Numerous battery safety precautions have been developed. High temperature shut down separators are designed to prevent thermal runaway and explosion. In an exemplary configuration, the separator includes an inner porous layer having a first melting temperature surrounded by an outer porous layer having a second melting temperature that is lower than the first melting temperature. For example, the inner porous layer can be poly propylene and the outer porous layer can be poly ethylene. If the battery cell is short-circuited and begins to increase in temperature, then the outer porous layer melts when the temperature reaches the first melting temperature. Melting of the outer porous layer fills the holes in the outer porous layer as well as some or all of the holes in the inner porous layer. Filling the holes stops the chemical reaction within the battery cell.
A polymeric positive temperature coefficient (PTC) device is a disk like device that can be connected to the positive electrode of the battery. The PTC device has the characteristic that if the temperature exceeds a threshold temperature, the electrical resistance of the PTC device increases several orders of magnitude, which inhibits current flow through the device. In this manner, the PTC device functions as a type of circuit breaker. When the current flowing through the device exceeds the current limit, the temperature increases above the threshold temperature, thereby greatly increasing the electrical resistance and inhibiting current flow.
In some configurations, a vent valve is incorporated into the battery housing. The vent valve is connected to the electrode via a conducting wire. If the internal pressure within the battery cell rises above a threshold pressure, then the vent valve blows, which physically separates the conducting wire from the electrode, thereby disabling current flow out of the battery. This is another form of a current interrupt device (CID).
Each of these safety precautions is effective when applied to a single battery cell.
However, additional considerations are necessary when addressing battery packs. A battery pack is a connected set of battery cells. Battery cells can be configured in series, parallel, or a mixture of both to deliver the desired voltage, capacity, or power density. Components of a battery pack include the individual battery cells and the interconnects which provide electrical conductivity between them. In many battery packs, current collector plates are used to collect the current output from each of the battery cells in the battery pack. A first current collector plate is connected to the anodes of each of the battery cells, and a second current collector plate is connected to the cathodes of each of the battery cells.
If one of the battery cells in a battery pack becomes faulty, such as short-circuiting, the faulty battery cell begins to increase in temperature. The battery cell can include any of the safety precautions described above to prevent further internal reaction and current flow within the faulty battery cell. However, in the battery pack, the faulty battery cell remains connected to active battery cells via the current collector plates. Current output from the active battery cells will flow to the faulty battery cell since the faulty battery cell is essentially a short within the battery pack circuit. This results in a large amount of current flowing into the faulty battery cell, which may lead to thermal runaway and a potential explosion. To prevent current from active battery cells from flowing into a faulty battery cell, the faulty battery cell needs to be electrically isolated.
A fusible link is a type of electrical fuse. At least one electrode of a battery cell is connected to a current collector plate via a fusible link. The fusible link functions as a current interrupt device between the battery cell and the current collector plate. The fusible link is typically a short piece of relatively thin metal wire or strip that melts when excessive current is applied, which interrupts the connection between the battery cell and the current collector plate. Short circuit, overload, or device failure is often the reason for excessive current. The size and construction of the fusible link is determined so that the heat produced for normal current does not cause the wire to melt and open the circuit.
A method of forming fusible links between battery cell electrodes and current collector plates is via a wire bond. Each bonding wire function as a fusible link and opens (melts) under excessive current, thereby disabling current flow through the faulted battery cell and electrically isolating the faulted battery cell from the active battery cells in the battery pack. However, the wire bonds are very fragile and are rigidly attached to the battery cell and the current collector plate. Under shock and vibration load, especially that seen in electric vehicles utilizing battery packs, the wire bonds are prone to breakage. One approach is to use adhesive to firmly attach the battery cells to the current collector plates to minimize relative motion between the two. This adds manufacturing cost and complexity. Moreover, this approach loses the ability to service individual battery cells since all battery cells are permanently attached to the current collector plates and cannot be reworked.